1. Field of the Invention
The present invention is directed to a method and a system for detecting nitrogenous explosives or narcotics by nuclear quadrupole resonance (NQR).
2. Description of the Prior Art
In order to limit the unrestricted flow of explosives and narcotics, it is desired to detect sub-kilogram quantities of those materials in monitoring stations. Most military explosives and narcotics share common features: they are crystalline solids containing nitrogen. Presently, the explosive detections system and methods cannot reliably detect sub-kilogram quantities of military explosives against a background of more benign materials.
In conventional vapor-based systems, dynamites and contaminated explosives may be detected. However, military explosives such as RDX and PETN are not reliably detected by the conventional vapor base systems especially when countermeasures are taken to reduce the effluent vapor and particles. Thermal neutron systems, which are .sup.14 N detectors, can detect relevant quantities of explosives. Unfortunately, conventional thermal neutron analysis systems frequently alarm on nitrogen-containing plastics. High false alarm rates are produced for inspected bags containing a few bomb equivalents of nitrogen in a benign form since the conventional thermal neutron analysis systems are sensitive only to the nuclear cross sections and not to any details of the particular chemical environment of the detected nitrogen nucleus. Hence the false alarm rate is inherently high, even with some spatial discrimination. Also, nuclear magnetic resonance (NMR) has been considered for detecting explosives. Because a large magnetic field is conventionally required for NMR, magnetically recorded data would be undesirable altered and other magnetizable materials could be damaged. Furthermore, the conventional non-vapor methods and systems are not suitable for inspecting people.